1. Field of the Invention
This invention relates to electric arc discharge lamps containing light emitting metals the desired radiation of which is in the visible region of the spectrum.
2. Description of the Prior Art
It is known, in the field of electric discharge lamps, to employ filters or reflectors which are transparent to desired visible radiation and reflective of undesired radiation, e.g., infrared radiation. See, for example, U.S. Pat. Nos. 3,221,198, 3,400,288 and 3,662,203 which disclose the use of thin metal layers, tin oxide coatings and titanium oxide coatings for such purposes.
Such reflectors are generally broad band; they improve lamp efficiency by returning the reflected radiation to the arc tube wall, thereby reducing the amount of power required to maintain a predetermined minimum wall temperature. However, such reflectors do not efficiently reflect radiation directly back to the arc plasma itself, that is, radiation to which the arc plasma is optically thick.
It is an object of this invention to provide a high intensity discharge lamp having a reflective filter which improves lamp efficiency more than prior art broad band reflectors do, by reflecting particular undesired radiation directly to the arc plasma.